The last few blocks of internet addresses using IPv4 are widely expected to be handed out this week. Southampton University's Tim Chown explores what happens next with the switch to IPv6.

As I write, the Internet Assigned Numbers Authority (Iana) has just allocated the IPv4 prefixes 39/8 and 106/8 to the Asia Pacific Regional Internet Registry (Apnic). Ordinarily, another couple of blocks of IPv4 address space disappearing off the shelf isn't headline news. This time, it's different.

These allocations mean the Iana has only five /8 blocks left, which, by prior agreement, will now be handed out one per regional internet registry (RIR). In plain English, the internet has run out of address space.

That sounds dramatic, but what does it actually mean? Well, the good news is the internet isn't going to stop working overnight. A lot of addresses allocated to ISPs and end sites remain as yet unused, so there's still breathing room. The Internet Engineering Task Force (IETF) has already produced IPv4's successor, IPv6. There's just the small detail that IPv6 deployment is described as, at best, in its infancy.

A unique problem

The lifetime of IPv4 has been prolonged largely thanks to network address translation (NAT). This has proliferated because it's easy to deploy and transparent to most users, who don't care about complexity in applications or the network, or that NAT makes initiating communications into their home networks more complex. If they can access Facebook, watch iPlayer and Google stuff, what's the big deal?

Vint Cerf has proposed that the UK government provide incentives to help businesses prepare for the imminent exhaustion of IPv4 addresses. Photo credit: David Meyer

Well, the problem is we need more unique IP addresses to support internet growth. This is growth of two types: innovation in devices, services and applications, as championed by the current 'internet of things' research agenda; and in countries that are still growing basic internet infrastructure. The gloomy alternative is increased address sharing, some customers running behind two layers of NAT and extensive use of carrier-grade NATs.

ISPs and organisations will now also have to trade for IPv4 blocks, and there may be pressure for some to return unused addresses. Trading will inevitably lead to more fragmented address space, larger routing tables and possibly poorer accountability regarding which organisations are using which addresses. It will certainly be interesting to see how the market value of IPv4 blocks shapes up. What price for a /12 with one careful owner?

Fortunately, the inevitable exhaustion of IPv4 was foreseen by the IETF, which since 1998 has been publishing IPv6 specifications. As a result of its work, IPv6 support is now present in all common router and operating system platforms, and Windows 7, Linux and Mac OS X have IPv6 enabled by default. Support in mobile and other devices is following fast.

IPv6 provides 128-bit address space, more than enough for the foreseeable future. While many benefits have been touted for IPv6, its one key feature is the hugely increased address space. Problem solved? Well, yes and no. IPv6 is the future, but today's internet is IPv4-based, and we need a way to introduce IPv6 without adversely affecting existing IPv4 services.

The ISPs won't deploy without IPv6 content, and the content providers argue no ISPs offer IPv6 to their customers. Something needs to change.

People talk of transition to IPv6, but coexistence is a better term. There's no flag day, and IPv4 will stick around for a long time to come.

Choices when switching to IPv6

While IPv6 is not directly interoperable with IPv4, being IP it runs under TCP/UDP and over the same link layers as IPv4. Through updates to APIs, network stacks and routing protocols, IPv6 can be run alongside IPv4 on the same infrastructure — a technique known as 'dual-stack'. This is the most common way IPv6 is being introduced today. It means devices on your network can communicate natively to external IPv4 or IPv6 systems without any form of translation required. You can also roll out dual-stack incrementally.

The alternative to running dual-stack is to operate an IPv6-only network. While this may be the long-term goal for many sites, it requires...